Macroscopically, rough surfaces wear more quickly and dissipate higher thermal energy due to friction compared to smooth surfaces. On the atomic scale, a material's surface structure largely controls its functional properties such as wetting, adhesion, adsorption, scattering, and chemical reactivity. (J. Aizenberg, A. J. Black, and G. M. Whitesides., Nature 394 (6696), 868-871 (1998); M. Gleiche, L. F. Chi, and H. Fuchs, Nature 403 (6766), 173-175 (2000); and D. Y. Ryu, K. Shin, E. Drockenmuller et al., Science 308 (5719), 236-239 (2005), the disclosures of each of which are incorporated herein by reference.) In nano-devices fabricated using bio-molecules, DNA, self-assembled monolayers, nanowires, and nanoimprinting, surface roughness has been found to be the main cause of decreased circuit yield, low device reliability, and scattering losses. (M. S. Islam, Z Li, S. C. Chang et al. Dramatically Improved Yields in Molecular Scale Electronic Devices Using Ultra-smooth Platinum Electrodes Prepared By Chemical Mechanical Polishing. 2005 5th Ieee Conference on Nanotechnology vol. 1, 80-83 (2005); and A. M. Agarwal, L. Liao, J. S. Foresi et al., Journal of Applied Physics 80 (11), 6120-6123 (1996), the disclosures of each of which are incorporated herein by reference.) In addition, the emerging field of plasmonic devices requires patterned metal films without unwanted roughness that can cause scattering or absorption of plasmons, degrading the device performance. (P. Nagpal, N. C. Lindquist, S. H. Oh et al., Science 325 (5940), 594-597 (2009), the disclosure of which is incorporated herein by reference.) Ultraflat surfaces are also important in reliable data storage media. (A. Khurshudov and V. Raman, Tribology International 38 (6-7), 646-651 (2005), the disclosure of which is incorporated herein by reference.) Thus, a wide range of applications would benefit from a material and an associated high-throughput process capable of yielding smooth and nano-patterned surfaces in a single step.
Typical surface roughness values that can be obtained for metals by polishing without resort to special equipment range from 25 to 500 nm. (N. J. Brown, Annual Review of Materials Science 16, 371-388 (1986), the disclosure of which is incorporated herein by reference.) A special combination of chemical and mechanical polishing (CMP) designed for single crystal semiconductors is usually not suitable for polycrystalline metals because metals are softer and the hard slurry particles damage the metal surface. (V. J. Logeeswaran, M. L. Chan, Y. Bayam et al., Applied Physics a—Materials Science & Processing 87 (2), 187-192 (2007), the disclosure of which is incorporated herein by reference.) Thin films are often smoother than bulk materials, but their residual roughness depends on the thickness of the film and the deposition temperature. (M. Higo, K. Fujita, Y. Tanaka et al., Applied Surface Science 252 (14), 5083-5099 (2006), the disclosure of which is incorporated herein by reference.) Even ultra thin films deposited at low temperatures exhibit non-negligible roughness values and have a limited physical stability.
Recently, a template-stripping technique has been shown to produce much smoother surfaces. (M. Hegner, P. Wagner, and G. Semenza, Surface Science 291 (1-2), 39-46 (1993), the disclosure of which is incorporated herein by reference.) Although this technique can significantly reduce the surface roughness, the intrinsic roughness due to the polycrystallinity of films imposes an ultimate limit. Moreover, the control of roughness and patterning on non-planar complex surfaces is difficult to achieve using these techniques. This is also true for single crystals, which can be atomically smooth but can only be grown from a limited range of materials under stringent conditions.
Bulk metallic glasses (BMGs) can be prepared from a wide range of chemical compositions and they display high strength and elasticity as a consequence of their amorphous structure. (A. L. Greer, Science 267 (5206), 1947-1953 (1995); A. Inoue, Acta Materialia 48 (1), 279-306 (2000); W. H. Wang, C. Dong, and C. H. Shek, Materials Science & Engineering R-Reports 44 (2-3), 45-89 (2004); and C. A. Schuh, T. C. Hufnagel, and U. Ramamurty, Acta Materialia 55 (12), 4067-4109 (2007), the disclosures of each of which are incorporated herein by reference.) These BMG materials have also gained significant scientific and technological interest due to their unique combination of mechanical properties and their amenability to novel processing techniques. A property unique among metals is that they exhibit a supercooled liquid region, a temperature region where the metallic glass first relaxes into a supercooled liquid before it eventually crystallizes. This unique softening behavior has been utilized for thermoplastic forming, (TPF) a processing method similar to the one used for plastic processing. (J. Schroers, Advanced Materials, 2010, 22: p. 1566-1597, the disclosure of which is incorporated herein by reference.) Various processing methods have been suggested based on TPF including extrusion, compression moulding, blow moulding, micro and nano-imprinting. As a method to fabricate solid complex 3D parts compression moulding has been explored. Typically in compression moulding the material is positioned in the mould cavity and the mould is closed to fill the entire cavity. This method has proven to be very efficient with plastics and was also explored for BMGs.
Metals exhibit high-energy surfaces and thus act as favourable sites for oxides and other surface contaminants, particularly at elevated temperatures. As a consequence, even BMGs, with a liquid-like structure, are microscopically rough in the as-prepared state. Additional surface roughness may originate from processing techniques such as casting, cutting, machining, and grinding etc. This starting roughness remains a part of the final BMG structure when fabricated by typical TPF methods because of the initial contact-area between the mould and the BMG. Although BMGs have shown self smoothening behaviours in the SCLR, the time scale on which it occurs can be longer than the desired forming time. Typical TPF time of 1-3 min is insufficient to smoothen features larger than 5 μm by surface tension alone. Additionally, any oxides which exist prior to or appear during processing remain solid and inhibit this phenomenon. Accordingly, a need exists for improved methods of forming BMGs.